1. Field of the Invention
The present invention relates to a power reception device and a power transmission system.
2. Description of the Related Art
In recent years, a contactless power transmission method (also referred to as wireless power transmission method) which enables power transmission without a contact of a metal portion by utilizing electromagnetic induction has attracted lots of attention. Japanese Laid-Open Patent Application Publication No. 2008-206325 discloses a prior art of such a contactless power transmission method. This prior art discloses that a power transmission device converts clock signals at a power transmission side generated by frequency modulation using binary data signals corresponding to two levels, “0” and “1,” respectively, into AC power (power transmission wave) and transmits the AC power (power transmission wave) to the power reception device. The power reception device includes a counter which operates in response to a clock signal at a power reception side having a higher frequency than the clock signal at the power transmission side. A time corresponding to cycles of the clock signal at the power transmission side is measured in such a manner that the counter counts the number of clock signals at the power reception side. Based on the measured number of the clock signals at the power reception side, frequencies of the clock signals at the power transmission side are detected.
FIG. 12 is a block diagram showing the conventional power reception device. Turning to FIG. 12, the power reception device includes a secondary coil L2, a power reception unit 42 including a rectification circuit 43, a load modulation unit 46, a power feeding control unit 48, and a power reception controller 50. The power reception device supplies electric power to a load 90 including a charging control unit 92 and a battery 94. An induced voltage at one end of the secondary coil (L2) is divided by a voltage division ratio between a resistor RB1 and a resistor RB2, and the divided voltage is input to a non-inverting input terminal of a comparator 71. The comparator 71 serves as a waveform shaping circuit. The comparator 71 outputs a clock signal (COMPI) at a power transmission side to a frequency binary data detection circuit 60. The frequency binary data detection circuit 60 includes a counter 73, a counter 77, a memory 79 and a f1/f2 determination circuit 81.
The frequency binary data detection circuit 60 measures a time corresponding to a n-cycle clock signal (COMPI) at a power transmission side by using an oscillation clock signal CLK (clock signal at the power reception side) of an oscillation circuit 58, and directly detects a frequency of the clock signal (COMPI). To be more specific, the counter 77 is activated in response to a first clock signal (COMPI) at the power transmission side, and starts counting using the clock signal CLK at the power reception side. When the counter 73 detects n cycles of the clock signal (COMPI) at the power transmission side, it outputs a detection signal CT to a reset terminal of the counter 77. The detection signal CT is fed to a memory 79 as a latch clock signal. In other words, the counter 77 is reset by the detection signal CT. A counted value of the counter 77 in reset is latched by the memory 79. In the above described manner, the cycles of the clock signal CLK at the power reception side, corresponding to n cycles of the clock signal (COMPI) at the power transmission side are measured, and the measured value is latched by the memory 79. The f1/f2 determination circuit 81 compares the measured value latched by the memory 79 to reference time information pre-obtained, to determine whether the frequency of the clock signal at the power transmission side is f1 or f2.
In the configuration of the above stated prior art, in the power reception device, since the number of clock signals at the power transmission side is counted, the clock signal at the power reception side generated by a clock signal source (oscillation circuit) which is different from the clock signal at the power transmission side is used. Therefore, it is difficult to establish synchronization between the power transmission device and the power reception device, and therefore it is difficult to cause a timing at the power transmission side and a timing at the power reception side to conform to each other in data communication between the power transmission side and the power reception side.